1. Heat the oil in a frying pan, add the onion, then fry for 5-8 minutes until lightly browned. Stir in the tomatoes, sugar, thyme and soy, then bring to the boil.
2. Simmer 5 mins, then slip the cod into the sauce. Cover and gently cook for 8-10 mins until the cod flakes easily. Serve with baked or steamed potatoes.
The sauce for this dish can be made up to a day ahead. Simply cool and chill until ready to reheat.